How does Java ConcurrentHashMap guarantee thread safety and expansion safety?

How does ConcurrentHashMap ensure thread safety

ConcurrentHashMap is a thread-safe hash table implementation provided in the Java concurrent package (java.util.concurrent). It ensures thread safety through the following features:

1. Segmented Lock:
   ConcurrentHashMap internally divides the hash table into multiple segments (segments), and each segment has an independent lock. Different threads can access different segments at the same time, reducing race conditions. This improves concurrency performance because different threads can modify different segments at the same time without blocking each other.

2. Atomic operations:
   Some operations in ConcurrentHashMap (such as put and remove) use atomic operations to ensure thread safety. These atomic operations are implemented through the underlying atomic operation instructions to ensure that concurrent access in a multi-threaded environment will not cause data inconsistency.

3. Memory visibility:
   ConcurrentHashMap uses the volatile modifier to ensure memory visibility. In a multi-threaded environment, modifications to ConcurrentHashMap will be seen by other threads immediately, avoiding data inconsistency.

4. Concurrency level:
   ConcurrentHashMap can control the number of internal segments (segments) by setting the concurrency level. Concurrency level refers to the number of threads that can modify ConcurrentHashMap at the same time. The higher the level of concurrency, the greater the number of segments, thereby reducing the competition between threads and improving concurrent performance.

In general, ConcurrentHashMap ensures thread safety through mechanisms such as segment locks, atomic operations, memory visibility, and concurrency levels. It can efficiently perform concurrent access in a high-concurrency environment, and provides an interface and function similar to HashMap.

How does ConcurrentHashMap ensure expansion security

When ConcurrentHashMap expands, it will adopt some strategies to ensure the security of expansion, so as to avoid data loss or deadlock and other problems. The following is the key mechanism for ConcurrentHashMap to ensure the security of expansion:

1. Segment lock guarantees concurrent expansion:
   ConcurrentHashMap internally uses a segment lock (Segmented Lock) mechanism. During expansion, the original hash table is divided into more segments, and each segment has its own lock. In this way, when expanding, only the affected segment will be locked, and other segments can still be accessed concurrently. This can reduce the granularity of locks, improve concurrency performance, and avoid locking the entire hash table during capacity expansion.

2. Lock-free read operation:
   During the expansion process of ConcurrentHashMap, the read operation is lock-free. Even in the process of expansion, other threads can continue to read the data of the hash table without being blocked. This ensures that the performance of read operations will not be affected during the expansion process.

3. Data migration ensures integrity:
   ConcurrentHashMap will migrate the data in the original hash table to the new hash table during capacity expansion. During the migration process, some technical means will be used to ensure the integrity of the data and avoid data loss or repeated migration. A common method is to use the CAS (Compare and Swap) operation to ensure the atomic operation of data and ensure the correct migration of data.

4. Control expansion trigger timing:
   ConcurrentHashMap will decide when to trigger the expansion operation based on the load factor (load factor) and the current number of elements. The load factor is a proportional value that measures how full the hash table is. When the filling degree of the hash table reaches a certain threshold, ConcurrentHashMap will trigger the expansion operation. By controlling the timing of expansion triggers, frequent expansion operations can be avoided and performance can be improved.

In general, ConcurrentHashMap guarantees the security of expansion through mechanisms such as segment locks, lock-free read operations, data migration guarantees, and control of expansion trigger timing. These mechanisms ensure that during the expansion process, problems such as data loss and deadlock will not occur, and the impact on concurrent operations is minimized.